In the factories of producing chicks including broilers, efforts for improvement have been made for enhancing the productivity of hatched chicks until they grow to a stage suitable for eating. For example, in chickens, it is tried to accelerate the growth by adding various nutrition-supplements such as vitamins to feeds.
Moreover, since the size of chicks hatched from fertilized eggs and the growth rate after hatch are generally proportional to the size and protein amount of fertilized eggs, much efforts for improving nutrition of parent chickens have been made for the purpose of producing fertilized eggs having a large size and containing a large amount of proteins. However, an egg-production ratio is generally low in the parent chickens laying fertilized eggs having a large size and containing a large amount of proteins. Therefore, the feeding cost for the parent chickens cannot be disregarded, which is required for securing regularly a large number of such fertilized eggs having a large size and containing a large amount of proteins.